1. Field of the Invention
The present invention relates to a method and an installation for shaping metal strip in a hot strip rolling mill which includes a finishing train, a cooling line, a pinch roll unit, and a coiler.
2. Description of the Related Art
It is known in the art to roll metal after the casting process in a hot strip rolling mill until it reaches a certain thickness in order then to convey the resulting product after a coiling procedure to a cold strip rolling mill for rolling the product to the final dimension. In this connection, increasingly higher demands are made of the metal strip supplied to the cold strip rolling mill with respect to its mechanical and geometric properties, particularly its flatness.
Simultaneously, there is the tendency that the desired final properties of the metal strip, which result from the successively arranged processes of the hot and cold rolling mills, is already adjusted in the hot rolling process or that a hot strip is produced which meets optimum requirements for the subsequent cold rolling process. Simultaneously, the boundary conditions become more difficult during hot rolling. Increasingly thinner and wider products are desired for adjusting them to the final product; this requires a greater thickness reduction and the use of greater rolling forces in the end stands of the hot strip rolling mill train. Consequently, the wear of the rolls increases with the decreasing size of the roll gap. Moreover, the thermal crown of the rolls increases when the production on the rolling train is increased. These effects negatively influence the flatness of the hot strip and, thus, also the quality of the strip in the cold state.
A conventional means for producing flat hot strip is the use of adjusting actuators. However, in the case of extremely thin strips, currently hardly any or no reliable hot strip planarity measurements are available.
Moreover, in a hot strip rolling mill, in addition to the deviations from flatness resulting from the finishing train, flatness changes of the metal strip occur in the cooling line and are caused by the pinch rolls.
The strip leaves the finishing train partially with a non-uniform flatness or stress distribution over the strip width. Even in the case of equal boundary conditions, such as, geometric dimensions, tensions, temperatures, material, etc., this may have the result in strips which are rolled in rapid sequence that different flatnesses in the cold strip are produced. This uneven flatness distribution of the hot strip then results directly or indirectly in different flatness conditions of the cold strip because of changed coiling conditions at the coiler, for example, higher coil crown.
Furthermore, the strip planarity changes due to the deflection of the strip at the pinch roll unit in the direction of the coiler by the different tensile stress distribution over the width of the strip. Principal influencing variables in this connection are the ground pinch roll shape, the wear of the pinch rolls, the contact pressure as well as the thermal crown of the pinch rolls. However, an optimization of the surface of the pinch rolls resulting from grinding as well as a change of the pinch roll material and the manner of exchanging the pinch rolls can improve the boundary conditions.
When the strip is wind into a coil, a non-uniform tensile stress distribution is partially produced over the strip width. In dependence on the tensile stress level, this non-uniform distribution produces different strip elongations over the strip width and, thus, non-planarities for the cold state. In this connection, principal influencing variables are the coil crown which is being adjusted. The shape of the coil depends on the strip contour, the strip flatness during coiling, the material strength (temperature, material quality) and the coiler tension.
A disadvantage is the fact that it is not possible to directly influence the changes of the planarity at the pinch roll as well as at the coiler and the changes caused by the winding of the coil. This produces local unflatnesses of the strip. Also, unsteadiness of the immediate strip end has been observed.